The Christian militia called Lebanese Forces has sworn to American special envoy Philip C. Habib that it will not enter the mainly Moslem sector of West Beirut when the Palestine Liberation Organization is withdrawn, a militia leader said today.

Fear of the Christian militia among the Moslem population is a sensitive point in the negotiations over the deployment of a multinational peace-keeping force on this capital's streets between the departing PLO guerrillas and the mostly Christian section of East Beirut.

"We have given a guarantee" to Lebanese President Elias Sarkis and Habib "that the Lebanese Forces will not enter West Beirut," said Karim Pakradouni, one of several top political advisers to rightist Phalange Party leader Bashir Gemayel.

"It is not our role to guarantee the security of West Beirut, and this decision is definitive," Pakradouni said in an interview today.

Since the 1975-1976 Lebanese civil war, Beirut has been divided into Moslem and Christian sections, each with its own sectarian-based or ideologically motivated militias. While Moslem and Christian civilians have lived on both sides of Beirut since the war, the PLO fighters, occupying Syrian troops and Moslem militias have stayed in West Beirut while the Christians and the rightist militias control East Beirut.

One of the PLO's arguments against a quick pullout from West Beirut has been the fear expressed by Palestinian and Lebanese Moslem leaders that the Lebanese Forces would invade West Beirut to settle old scores growing out of Lebanon's seven-year-old domestic conflict. A principal purpose of the multinational force, to include U.S., French and Italian troops, is to prevent such an incursion and alleviate these fears.

Asked several ways if the Lebanese Forces intend "never" to go into West Beirut, Pakradouni several times responded, "never." The interview took place the day after the weekly meeting of the Phalangist Party's political bureau, of which Pakradouni is a member.

Presidential candidate Gemayel is also commander of the Lebanese Forces. Last month his effort to effect a reconciliation with one of West Beirut's key leftist Moslem leaders, Walid Jumblatt, collapsed after Jumblatt called Gemayel an Israeli-supported candidate, according to a Lebanese Forces official.

Pakradouni said the Phalangists favor the deployment of the recently rebuilt 20,000-man Lebanese Army to replace the multinational peace-keeping force. "The multinational army is only for the departure of the PLO," he said.

But national Liberal Party secretary general Dory Chamoun disagreed in a weekend interview, indicating that he feels the Lebanese Army is still too weak to take on any strenuous peace-keeping roles by itself.

"We would like to see the United States sponsor a multinational force to remain in Lebanon for one or two years until such time as the Lebanese Army reconstitutes itself," Chamoun said.

Chamoun, who returned to Lebanon 10 days ago after almost two months in Washington, said he had requested such an extension of a U.S.-sponsored force in talks with Reagan administration officials.

The Lebanese Army split along Moslem-Christian lines during the second half of the civil war in 1976. Since then, the Army has fallen apart in the same manner three more times and just recently was rebuilt. Today, it has a 50-50 Christian-Moslem rank and file and a "thoroughly mixed" Christian-Moslem high command, according to a Western military analyst.